It is useful to classify my research into three projects. The objective of the first project is to develop a method by which every essential irremediable gene (whose mutation is not remediable by some component in a complete medium) in E. coli can be identified by recessive lethal mutations, the lethality of which is normally masked by an F-merogenote. These recessive lethal mutations are to be classified by complementation tests. The second project involves the isolation of mutants with increased (and decreased) rates of spontaneous and acridine-induced frameshift mutation. The relationships between frameshift mutator and (antimutator) activity, base substitution mutator activity, DNA replication and repair, and spontaneous deletion formation activity will be studied. The third project is concerned with the elucidation of the biochemical genetics and regulation of the metabolic steps that start with the ribonucleoside monophosphates and end with the biosynthesis of RNA.